<!DOCTYPE html>
<html>
  <head>
    <meta charset="UTF-8">
    <title>计算圆的周长和面积</title>
  </head>
  <body>
    <div>
      <p>圆的半径：<input id="r" type="text"></p>
      <p>圆的周长：<input id="cir" type="text"></p>
      <p>圆的面积：<input id="area" type="text"></p>
    </div>
    <script>
      var r = prompt('请输入圆的半径');
      r = parseFloat(r) && Number(r);   // 获取输入的纯数字，其余情况皆转为NaN
      if (!isNaN(r)) {                  // 判断用户的输入是否是数值
        var cir = 2 * Math.PI * r;
        var area = Math.PI * r * r;
        document.getElementById('r').value = r;
        document.getElementById('cir').value = cir.toFixed(2);
        document.getElementById('area').value = area.toFixed(2);
      } else {
        alert('请输入正确的数字');
      }
    </script>
  </body>
</html>